Heretofore, lead bronze or phosphorus bronze has been used as the above-mentioned sliding materials. In addition, proposals have been made to enhance the seizure resistance of these copper alloys, such as by adding the matrix-strengthening elements, such as P, Al and the like, or adding Bi or the like which has good compatibility. These proposals have achieved some success.
However, when the lead bronze or phosphorus bronze is used in the lubricating oil having a high sulfur content and at a high temperature, black copper sulfide is formed on the surface. Since this copper-sulfide layer is of low-strength and is not tightly bonded to the substrate, such layer easily peels off from the substrate. It has consequently, turned out that seizure or abnormal wear thus occurs. Furthermore, it has also turned out that the copper alloys are corroded by the sulfur components of the lubricating oil, which leads to decrease in strength of the material and hence accelerates fatigue. In addition, the seizure of the lead bronze and the phosphorus bronze has been readily incurred under such sliding condition which brings about a mixture or boundary lubrication due to the small amount of lubricating oil.
Engine oil, transmission oil, gear oil or the like are the lubricating oil which lubricates between the above-mentioned sliding materials and the opposite material. The sulfur-based additives are added to these oils.
First, the additives to the gasoline-engine oil are dialkylmonosulfide for preventing the oxidative deterioration of the engine oil, a sufonate- or phenate- base metal detergent agent for cleaning the sludge formed by oxidation of the engine oil, dithiophosphate molybdenum compound or dithiocarbamate molybdenum compound for preventing foaming of the low-viscosity engine-oil, or the like. The above-mentioned dialkylmonosulfide is believed to ionically decompose the hydroperoxide which is formed by the oxidation of the base oil. However, detrimental effects of the additives are also pointed out. For example, the metal-based detergent agent forms sulfate-ash or sludge. The amount of its use is therefore limited. In addition, it is said that the foaming-preventing agent may exert a detrimental effect on the performance of the bearing metal.
ZnDTP (dialkyldithiozincphosphate) is added to the diesel-engine oil as a countermeasure against the wear due to soot. Into the rotary-engine oil, olefin sulfide, sulfurized oil or the like is added as the sulfur-based extreme pressure agent, and thiozinc-phosphate, dithiocarbamate molybdenum sulfide is added as the organic-metal anti-wear agent.
Into the transmission oil and the gear oil, olefin sulfide, sulfurized oil or the like is added as the sulfur-based extreme pressure agent, and thiozinc-phosphate or dithiocarbamate molybdenum sulfide is added as the organic-metal anti-wear agent, phosphate-esteramino-salt as the phosphorus-base anti-wear agent. The sulfur concentration ranges from 0.37 to 1.7% in the commercially available oil at present.
It is known that, when the above-mentioned various oils deteriorate, the oil incurs corrosion problems in the copper-based sliding materials. As a corrosion countermeasure, the present applicant filed the following patent applications.
U.S. Pat. No. 4,878,768: The deteriorated oil causes the corrosion of the Pb phase present in the clearances between the skeleton of the Cu--Pb based sintered alloy used as a sliding bearing of a diesel engine. In order to prevent such corrosion, In is added to the Pb phase.
Japanese Unexamined Patent Publication No. 7-118,777: sintered copper-alloy-based composite sliding member, which is based on the sintered copper alloy consisting of more than 15% and up to 40% of Zn, from 0.5 to 6% of graphite, and from 0.5 to 6% of one or more of Al.sub.2 O.sub.3, SiO.sub.2 and Fe.sub.3 P, and the balance being Cu. In this application, Zn is added in the above-mentioned amount to prevent the corrosion which results from the fact that the deteriorated transmission oil forms CuS on the surface of the copper alloy.
The present inventors carried out a sliding test of a copper-based sliding material, in which the lubricating oil with the above-mentioned sulfur-based additives was used, and which almost wholly reproduces the usage conditions of an actual machine. It was then recognized that the total acid value of the lubricating oil (diesel-engine oil, CD grade, 10W-30) greatly increased as follows.
______________________________________ Time 0 (h) 50 (h) 100 (h) 150 (h) 220 (h) ______________________________________ Total acid number 1.3 6.4 18.0 21.5 22.3 (mg KOH/g) Strong acid number 0 0 0.1 0.2 0.3 (mg KOH/g) Total base number 3.8 0.5 0 0 0 mg KOH/ ______________________________________
The conventional Kelmet and the copper-based sliding materials proposed by the present inventors exhibit unsatisfactory corrosion resistance in the deteriorated lubricating oil. The usage environment of sliding bearings, particularly, a piston-pin bush, a bush of automatic transmission or the like, is under increasing severity, which incurs the reaction between the copper and sulfur in the lubricating oil leading to formation of copper sulfide. The wear is aggravated, therefore. Further, because of considerable temperature-rise, the oil-film is cut on the sliding parts and the seizure problem arises.
The conventional lead bronze or phosphorus bronze does not corrode due to sulfur, when the lubricating oil is free of the sulfur-based additive or the amount of its addition is low. Enhancement of the seizure resistance of these bronzes is desired, however, even in such case.
It is an object of the present invention to provide a copper-based sliding material, which has improved resistance against the lubricating oil to which a sulfur-based additive has been added and which has been deteriorated due to use.
It is also another object of the present invention to enhance the seizure resistance of the copper-based sliding material which is lubricated by the lubricating oil which is free of the sulfur-based additives or in which the addition amount of the sulfur-based additive is small.
It is, furthermore, an object of the present invention to provide a surface treatment method which has an object to enhance the corrosion resistance of the copper-based sliding material against the deteriorated lubricating oil.